(1) Field of the Invention
This invention relates to a plasma display panel used as a display device and the method of producing the display panel, specifically to a plasma display panel suitable for a high-quality display.
(2) Description of the Prior Art
Recently, as the demand for high-quality large-screen TVs such as high-vision TVs have increased, displays suitable for such TVs, such as Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), and Plasma Display Panel (PDP), have been developed.
CRTs have been widely used as TV displays and excel in resolution and picture quality. However, the depth and weight increase as the screen size increases. Therefore, CRTs are not suitable for large screen sizes exceeding 40 inch. LCDs consume a small amount of electricity and operate on a low voltage. However, producing a large LCD screen is technically difficult, and the viewing angles of LCDs are limited.
On the other hand, it is possible to make a PDP with a large screen with a short depth, and 40-inch PDP products have already been developed.
PDPs are divided into two types: Direct Current (DC) and Alternating Current (AC). Currently, PDPs are mainly AC-type since they are suitable for large screens.
FIG. 1 is a sectional view of a conventional AC PDP. In the drawing, front cover plate 1, with display electrodes 2 put thereon, is covered by dielectric glass layer 3 which is lead glass, namely, PbO-B2O3-SiO2 glass.
Set on back plate 5 are address electrode 6, partition walls 7, and fluorescent substance layer 8 consisting of red, green, or blue ultraviolet excited fluorescent substance. Discharge gas is charged in discharge space 9 which is sealed with dielectrics glass layer 3, back plate 5, and partition walls 7.
The discharge gas is generally helium (He), xenon (Xe), or mixture of neon (Ne) and Xe. The amount of Xe is generally set to a range from 0.1 to 5% by volume, preventing the drive voltage of the circuit from becoming too high.
Also, the charging pressure of the discharge gas is generally set to a range from 100 to 500 Torr so that the discharge voltage is stable (e.x., M. Nobrio, T. Yoshioka, Y. Sano, K. Nunomura, SID94' Digest, pp. 727-730, 1994).
PDPs have the following problems concerning brightness and life.
Currently, PDPs for 40-42-inch TV screens generally have a brightness of about 150-250 cd/m2 for National Television System Committee (NTSC) standard (number of pixels being 640×480, cell pitch 0.43 mm×1.29 mm, square of one cell 0.55 mm2) (Function & Materials, Feb., 1996, Vol. 16, No. 2, page 7).
On the contrary, in 42-inch high-vision TVs, number of pixels is 1,920×1,125, cell pitch 0.15 mm×0.48 mm, and square of one cell 0.072 mm2. This square of one cell is 1/7-⅛ of that of NTSC standard. Therefore, it is expected that if PDP for 42-inch high-vision TV is made with the conventional cell construction, the screen brightness decreases to 30-40 cd/m2.
Accordingly, to acquire, in a PDP used for a 42-inch high-vision TV, the same brightness as that of a current NTSC CRT (500 cd/m2), the brightness of each cell should be increased about 12-15 times.
In these circumstances, it is desired that the techniques for increasing the brightness of PDP cells are developed.
The light-emission principle in PDP is basically the same as that in fluorescent light; a discharge lets the discharge gas emit ultraviolet light; the ultraviolet light excites fluorescent substances; and the excited fluorescent substances emit red, green, and blue lights. However, since discharge energy is not effectively converted to ultraviolet light and conversion ratio in fluorescent substance is low, it is difficult for PDPs to provide brightness as high as that of fluorescent lights.
It is disclosed in Applied Physics, Vol. 51, No. 3, 1982, pp. 344-347 as follows: in PDP with He-Xe or Ne-Xe gas, only about 2% of the electric energy is used in ultraviolet light, and about 0.2% of the electric energy is used in visible rays (Optical Techniques Contact, Vol. 34, No. 1, 1996, page 25 and FLAT PANEL DISPLAY 96, Parts 5-3, NHK Techniques Study, 31-1, 1979, page 18).
Accordingly, to increase light-emission efficiency is considered as important in increasing the brightness of PDP cells.
Now, regarding to the PDP life, the following are generally considered to determine the PDP life: (1) the fluorescent substance layer deteriorates since plasma is confined to a small discharge space to generate ultraviolet light; and (2) the dielectrics glass layer deteriorates due to sputtering by gas discharges. As a result, methods for extending the fluorescent substance life or preventing the deterioration of dielectric glass layer are studied.
As shown in FIG. 1, in conventional PDPs, protecting layer 4 consisting of magnesium oxide (MgO) is formed on the surface of dielectrics glass layer 3 with a vacuum vapor deposition method to prevent the dielectrics glass layer from deteriorating.
It is desirable that protecting layer 4 has high sputtering resistance and emits a large amount of secondary electrons. However, it is difficult for a magnesium oxide layer formed by the vacuum vapor deposition method to obtain a protective layer having enough sputtering resistance. There is also a problem that discharges decrease the amount of secondary electron emitted.